Transmission for a working vehicle

ABSTRACT

A housing of a transmission which can drive front wheels and rear wheels and a power take-off shaft is constructed having a first chamber and a second chamber. A hydrostatic transmission is housed in the first chamber. A gear-type speed changing unit is housed in the second chamber. A hydraulic pump and a hydraulic motor are separately disposed on the inner and outer surfaces of the front wall of the first chamber. A clutch mechanism for engaging or disengaging with or from the front wheels for supplying power thereto is also provided. On the housing is provided a restraint mechanism for restraining rearward acceleration more than a predetermined speed by a speed control lever of the hydrostatic transmission when the gear-type speed changing unit is switched to the high speed setting. Left and right running brake units provided on the vehicle are made independently operable by treading left and right turn brake pedals on a first brake pedal shaft and made simultaneously operable by treading a master brake pedal on a second brake pedal shaft.

FIELD OF THE INVENTION

The present invention relates to a transmission for driving front andrear wheels of a running vehicle. More specifically, the inventionrelates to, a transmission in which is provided in a housing unitthereof: a hydrostatic transmission (hereinafter referred to as an HST),a front wheel driving construction, a restraint mechanism for rearwardspeed change, a power transmission system linking an input shaft of apower take-off shaft of the transmission with a power take-off clutchand a power take-off braking unit, and a brake operating constructionwhich enables left and right braking units to provide oil to the vehicleindependently with left and right turn brake pedals or simultaneously bya master brake pedal.

BACKGROUND OF THE INVENTION

A transmission equipped with an HST and for driving front wheels andrear wheels is well-known, as disclosed in, for example, U.S. Pat. No.5,544,547. With this technique, an engine transmits power through aninput shaft to drive a hydraulic pump of the HST. The input shafttransmits the power from the rear end of the hydraulic motor of the HSTto a differential gear unit, which in turn drives a gear-type speedchanging unit. This unit drives the rear and front wheels through auniversal joint by a fore end of the output shaft forwardly projectingfrom a housing.

This technique is constructed so that both the hydraulic pump andhydraulic motor are housed in the housing and the front wheels aredirectly driven by the output shaft of the hydraulic motor of the HST.Thus, when the speed of the gear-type speed changing unit is changed,the speed ratio between the front wheels and the rear wheels is notsynchronized. In order to avoid this occurrence, a restraint mechanismmust be provided which restrains the front wheels from being driven whenthe gear-type speed changing unit is switched to the high speed setting.

The transmission provided with the HST and gear-type speed changingunit, when the latter is switched to the high speed operating position,cannot operate a speed change pedal of the HST to increase the speed ofrearward movement more than a predetermined value. This is well-knownin, for example, Japanese Utility Model Publication No. Sho 62-5942.

In such a conventional technique, the restraint mechanism for rearwardmovement speed change with respect to the speed changing unit of the HSTis constructed in such a manner that in order to change the speed of thegear-type speed changing unit, a restraint ring is interposed outside ofthe housing and between the operating lever and the speed changing pedalsituated outside the housing. Consequently, when dust or the likeattaches to the link after long use, complete restraint may not be made,resulting in a lack of reliability.

Also, the aforesaid U.S. Pat. No. 5,544,547 discloses that in a housingof a running vehicle are housed the HST, a power take-off (hereinafterreferred to as a PTO) clutch, and a PTO brake unit. In detail, thehousing is partitioned into a front chamber and a rear chamber. The HSTis housed in the front chamber. A PTO transmission for transmittingpower from an input shaft to a PTO shaft through a PTO transmissionshaft onto which the PTO clutch is disposed, and the PTO brake unit forstopping the rotation of the PTO shaft, are located in the rear chamber.The PTO brake is a hydraulic actuation-type which forms a cylinderchamber on the side wall of a housing of the rear chamber. Pistons arehoused in the cylinder chamber and a brake pad provided at the utmostend of a piston rod is pressed onto the outer peripheral surface of aclutch casing of the PTO clutch to thereby exert the braking action.

The brake pad, however, must strongly be pressed onto the clutch casing,whereby a large braking capacity is required to enlarge the PTO brakeunit. Furthermore, the cylinder for actuating the PTO braking unit mustbe enlarged. Since the cylinder chamber is in the rear housing, an oilpressure supply passage for making the brake unit inactive when the PTOclutch is "on", is required to lead from the front housing to the rearone. As a result, the brake response is delayed due to conduitresistance. If a larger amount of inertia is mounted on the vehicledriven by the PTO shaft, the PTO clutch, when put on, is abruptlyconnected and applies a large force to the PTO shaft and input shaft. Insuch cases, the engine experiences a large shock and may stop.

On a step at one lateral side of the vehicle is disposed a running pedalfor operating the HST which is forward-rearward-switchable. On the stepat the other lateral side are disposed two left and right turn brakepedals independently operable of the left and right brake units providedon the vehicle. One master brake pedal is disposed in the vicinity ofthe running pedal, which is well-known by, for example, Japanese PatentPublication No. Hei 6-20867.

In the above-mentioned technique, a support cylinder is laterally androtatably mounted to a support frame below the seat. The right turnbrake pedal is fixed to one end of the support cylinder and a firstouter cylinder is fixed to the other end. And, an abutting arm, attachedto the first outer cylinder is connected with the left and right brakeunits. A pedal shaft is relatively rotatably fitted into the supportcylinder. Onto one end of the pedal shaft is fixedly fitted a secondouter cylinder on which is mounted the left turn brake pedal. The secondouter cylinder connects with the left running brake unit. Thus, when theleft and right turn brake pedals are trod the left and right brakeunits, respectively, are independently exerted. The other end of thepedal shaft outwardly projects from the first outer cylinder andprovides the abutting arm. A boss portion of the master brake pedal isrelatively rotatably fitted onto the support cylinder adjacent to thefirst outer cylinder, and an interlocking member is provided on the bossportion, so that, when the master brake pedal is trod, the interlockingmember simultaneously abuts against the abutting arm of the first outercylinder and that of the brake shaft so as to simultaneously exert theleft and right brake units.

In order to stop a vehicle having a running pedal, left and right turnbrake pedals and master brake pedal, an operator changes his right foottreading upon the forward or rearward running pedal to the master brake.For a nimble foot change, the master brake pedal must be easy to operatewith respect to the respective running pedals. In the vehicle providedwith a locking mechanism capable of fixing and releasing the runningpedal in an optional forward movement operating position, when the leftand right brake its equipped on the vehicle are simultaneously actuated,the locking mechanism must simultaneously be released. It is suggestedthat the locking mechanism be released in association with treading themaster brake pedal. The conventional construction, however, sets theaxis of rotation of the master brake pedal necessarily on the axis ofrotation of the turn brake pedal. Therefore, the position of the masterbrake pedal is inevitably restricted to the running pedal or the lockingmechanism, when designing the layout of the transmission.

Furthermore, the support cylinder for fixing the right turn brake isrelatively rotatably fitted onto the outer periphery of the brake shaftto which the left turn brake is fixed. The cylinder fixing thereon themaster brake is relatively rotatably fitted onto the outer periphery ofthe support cylinder. This triple construction applies a frictionalforce on both the brake shaft and the cylinders, for example, when theright turn brake pedal is trod to rotate the cylinder. When the outsideair temperature is low and the viscosity of lubricating oil interposedbetween the brake shaft and the support cylinder and between the supportcylinder and the cylinder is high, the frictional force is enlarged. Asa result, the brake shaft and cylinder rotate together, raising theconcern that both brake units might malfunction, and cause the vehicleto stop, when the vehicle is intended to be turned rightward.

In the construction such that the pedal shaft mounting thereon the leftand right turn brake pedals and the pedal shaft for mounting thereon themaster brake pedal are coaxially disposed, when the master brake pedalis trod, the interlocking member of the master brake pedalsimultaneously strike the respective abutting arms of the left and rightbrake pedals. Therefore, in order to simultaneously operate the left andright brake units, the respective abutting arms and the interlockingmember must accurately be positioned for mounting. When parts areprocessed or assembled in error, the interlocking member may mis-timethe striking of the abutting arms. If the abutting arm, which strikesthe interlocking member, operates one brake unit in advance, the vehiclecannot be stopped in a straight line. Moreover, even if the abutting armis accurately positioned, the abutting arm or the interlocking membermay wear due to use for long time or excessive treading. Consequently,the aforesaid timing may go wrong, making it extremely difficult toprecisely adjust the brake unit.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a compact transmissionwhich can drive front wheels, rear wheels, and a PTO shaft. For thispurpose, first and second chambers are formed in the front and the rearof a housing. An HST is housed in the first chamber and a gear-typespeed changing unit is housed in the second chamber. Both members areconnected in a driving manner. On the inner surface of a front wallconstituting the first chamber is disposed a hydraulic pump and ahydraulic motor, constituting the HST. On the outer surface of the sameis disposed another hydraulic pump separate from the first. A vacancy inthe first chamber houses a clutch mechanism for engaging or disengaginga front wheel driving shaft with or from an output portion of thegear-type speed changing unit.

Another object of the invention is to restrain acceleration of rearwardmovement of the vehicle. Where the output rotation speed of the HST ischanged by the gear-type speed changing unit to the high speed setting,a restraint mechanism restrains the acceleration to a predeterminedspeed for rearward movement. The restrain is housed in the firstchamber, and makes rearward movement in the high speed settingimpossible.

Still another object of the invention is to form the front wall of thehousing so that the hydraulic pump and hydraulic motor are mounted onand separable from the unit.

A further object of the invention is to provide a PTO transmission. ThePTO transmission unit transmits power from an engine to a PTO shaftthrough a PTO clutch means. On a partition between the first chamber andthe second chamber is disposed a PTO brake unit, for braking the PTOshaft when the PTO clutch mechanism disconnects.

It is a still further object of the invention that left and rightrunning brake units, equipped on the vehicle, can independently exertthe braking action by treading left and right turn brake pedals,respectively. A first brake pedal shaft and a second brake pedal shaftare disposed longitudinally of the vehicle body and laterally rotatablythereof in order to make possible the simultaneous braking action bytreading a master brake pedal. At one lateral end of the second brakepedal shaft are juxtaposed turn brake pedals for desirable, independentbraking of the left and right running brakes. The master brake pedal isdisposed at the other lateral ends of the first brake pedal shaft. Atone lateral end of the first brake pedal shaft is provided asimultaneous brake operating mechanism with respect to a trip-turn brakepedal.

These and other objects, features and advantages of the invention willbecome more apparent upon a reading of the following detailedspecification and drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side view of a working vehicle incorporating the presentinvention;

FIG. 2 is a schematic diagram of the power transmission of a runningsystem and a PTO system of the present invention;

FIG. 3 is a cross-sectional side view of a front casing of the presentinvention;

FIG. 4 is a cross-sectional side view showing a gear-type speed changingunit and a power transmission mechanism to a mid-PTO shaft;

FIG. 5 is a cross-sectional side view of an upper portion of a housing,showing a power transmission mechanism to a front wheel driving shaftand a power transmission mechanism to a rear PTO shaft;

FIG. 6 is an enlarged cross-sectional view of a PTO brake and a PTOclutch mechanism;

FIG. 7 is an elevational view showing the positional relationshipbetween the running system and the PTO system;

FIG. 8 is a cross-sectional view looking in the direction of the arrowsX--X in FIG. 3;

FIG. 9 is a cross-sectional plan view of a speed changing portion of thegear-type speed changing unit and a rearward speed change restraintunit, engageable with the speed change operating portion;

FIG. 10 is cross-sectional views looking in the direction of the arrowsY--Y in the right side of FIG. 3 and a cross-sectional view looking inthe direction of the arrows Y'--Y' in the left side of FIG. 3;

FIG. 11 is a front perspective view of the housing;

FIG. 12 is a hydraulic circuit diagram;

FIG. 13 is an elevational view showing a side plate and the ambientportion thereof;

FIG. 14 is a side view of the side plate;

FIG. 15 is a rear view of the same;

FIG. 16 is a graph showing the relation between the shaft torque actingon a PTO transmission shaft and time;

FIG. 17 is an elevational view showing a restraint mechanism when aspeed control arm is in a neutral position;

FIG. 18 is an elevational view showing the restraint mechanism when thespeed control arm is in a maximum acceleration position at the forwardmovement side of the vehicle;

FIG. 19 is an elevational view showing the restraint mechanism when thespeed change arm is regulating rearward acceleration;

FIG. 20 is an elevational view showing the restraint mechanism when thespeed control arm is in the maximum acceleration position at therearward movement of the side brake;

FIG. 21 is an exploded view of a PTO brake;

FIG. 22 is a cross-sectional view looking in the direction of the arrowsZ--Z in FIG. 8;

FIG. 23 is a cross sectional view of a rear portion of a rear casing anda rear axle casing;

FIG. 24 is a cross-sectional plan view of a vehicle body framesupporting first and second brake pedal shafts;

FIG. 25 is a cross-sectional view of the vehicle body supporting a firstbrake pedal shaft;

FIG. 26 is a perspective view of a simultaneous brake operatingmechanism with respect to left and right turn brake pedals;

FIG. 27 is a left side plan view of the vehicle body frame;

FIG. 28 is a right side plan view of the same;

FIG. 29 is a plan view of the right side of the vehicle body frameshowing the support for forward and rearward running pedal shafts;

FIG. 30 is a partial cross-sectional plan view showing engagement of theforward and rearward running pedal shafts; and

FIG. 31 is a perspective view of a cruise control lever and alost-motion mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation will be given on the entire construction of a mid-mountmower type working vehicle in accordance with FIG. 1, in which: anengine E is mounted on a vehicle body frame in a bonnet 1 in the frontof the vehicle body; a dashboard 2 is disposed behind the bonnet 1; ahandle 3 is disposed on the dashboard 2; steps 4 are disposed at bothsides below and between the dashboard 2 and a seat 6; brake pedals 5Land 5R are disposed on the left side of step 4; a forward running pedal8F and a rearward running pedal 8R, to be discussed below, arejuxtaposed on the right side step 4; and at the lateral side of the seat6 are disposed a sub-speed-changing lever 7 and a PTO switching lever 9.

Under the seat 6 is disposed a transmission of the present invention.The transmission comprises a hydrostatic transmission (hereinafterreferred to as HST), a gear-type speed changing unit and a PTOtransmission unit housed in a front casing 10 and a rear casing 11. Aninput shaft 12, a front wheel driving shaft 13, and a mid-PTO shaft 14forwardly project from the front casing 10. With the input shaft 12 areinterlocked a crank shaft of the engine E through a damper and atransmission shaft 16 in a vehicle body frame 15. The front wheeldriving shaft 13 connects through a universal joint 17 with an inputshaft 19 projecting from the front axle casing 18 through a universaljoint 17. Thus, power is transmitted from the front wheel driving shaft13 to the input shaft 19 which in turn drives front wheels 20 journalledto the left and right sides of front axle casing 18. Reference numeral22 designates a hydraulic cylinder which expands and contracts byoperating a handle 3 so as to steer the front wheels 20. The mid-PTOshaft 14 interlocks through a universal joint 23 with an input shaft 26projecting from a gear box 25 of a mower 24, which is constructed toelevate and fall by a lift (not shown) with respect to the groundsurface in a space between the front wheels 20 and the rear wheels 21.

Rear axle boxes 127L and 127R are mounted at both side surfaces at therear of the rear casing 11. Axles 27L and 27R laterally project andpivotally support rear wheels 21 for driving. On the upper surface ofthe rear casing 11 is mounted a hydraulic cylinder casing 29 for ahydraulic lift. A pair of left and right lift arms 30 rearwardly projectfrom the cylinder casing 29. Lower links 31 are pivotally supported atthe front portions thereof onto the rear surface of the rear casing 11.A plate provided with a draw bar hitch 32 is mounted to the lower end ofthe same. A working machine is mountable in relation of freely raisingand lowering the lift arms 30, lower links 31 and top links (not shown),and a traction working machine is mountable to the draw bar hitch 32.

Next, explanation will be given on the structure of the transmission inaccordance with FIGS. 2 and 3, in which the front casing 10 and rearcasing 11 are longitudinally divisibly jointed to form the housing. Thefront casing 10 forms in the front surface thereof a front wall 10a andat the rear surface a rear wall 11b. A flange 10h for mounting thetransmission to the end of the body frame 15 is formed in the peripheryof the front wall 10a. Within the flange 10h is mounted a center section40 for closing an opening formed in the front wall 10a of the frontcasing 10. A first chamber R1 is formed between the rear wall 10b of thefront casing 10 and the center section 40. The center section 40supports a hydraulic pump P1 and a motor M, to be discussed below, andforms part of the front wall 10a. A bore for supporting a front wheeldriving shaft 13 is formed on the front wall 10a at the vertically lowerand about laterally center portion thereof. The rear casing 11 is openat the front end thereof and a partition 11a is provided at thelongitudinal center of the body so as to longitudinally divide theinterior of the rear casing 11. A second chamber R2 is formed betweenthe rear wall 10b of the front casing 10 and the partition 11a. A thirdchamber R3 is formed between the partition 11a and a rear wall 11c ofthe rear casing 11. Furthermore, a recess is formed on the outer surfaceof the rear wall 11c of the rear casing 11. The recess is closed by alid 79 so as to form a fourth chamber R4.

An input shaft 12 is supported at the upper portion of the first chamberR1 and between the center section 40 and the rear wall 10b. Thehydraulic pump P1 is disposed inside the first chamber R1 on the centersection 40. The input shaft 12 drives the hydraulic pump P1. Outside thecenter section 40 is disposed a pump casing 105. Inside the pump casing105 is a charging pump P2. The input shaft 12 penetrates the pump casing105 so that the input shaft 12 also drives the charging pump P2. A firstrunning transmission shaft 41 is supported in the first chamber R1 atsubstantially the longitudinal center. The hydraulic motor M, mounted onthe outside of the center section 40, is disposed coaxially of the firstrunning transmission shaft 41. The hydraulic pump P1 and hydraulic motorM are fluidly connected with each other through an oil passage bored inthe center section 40, to be discussed below, thereby constituting theHST. The hydraulic pump P1 and hydraulic motor M are mounted in aposition on the front wall 10a separate from the housing, therebyfacilitating the assembly of HST and of the front casing 10.

With reference to FIG. 2, the hydraulic pump P1 is constructed so that acylinder block 51 is rotatably disposed on a pump mounting surface 50formed on the upper, inner surface of the center section 40. The inputshaft 12 engages with the cylinder block 51 at the center thereof. Axialpistons 52 are housed in a plurality of cylinder bores in the cylinderblock 51. Axial pistons 52 are movable in reciprocation through biasingsprings, and abut at the heads against a thrust bearing 53a of a movableswash plate 53. The movable swash plate 53 can slant the abuttingsurface of the thrust bearing 53a from the vertical condition shown,around trunnions 53b projecting sidewardly from the movable swash plate53. As shown in FIG. 8, one trunnion 53b is rotatably supported at theinner surface of the left side wall of the front casing 10 and the otheris rotatably supported at the right side plate 60. Right side plate 60closes an opening formed at the right side wall of the front casing 10.Onto the outer periphery of the other trunnion 53b is fitted a neutralreturn spring 59 (refer to FIG. 22). Both ends of the neutral returnspring 59 extend in the same direction and cross each other so as tosandwich therebetween a movable pin 69 mounted on the side surface ofthe movable swash plate 53 and a fixed eccentric pin 78 mounted on theinner surface of the right side plate 60. Pin 78 is set in apredetermined position to return the movable swash plate 53 to theaccurate neutral position. The trunnion 53b projects through to theoutside of the right side plate 60 and a speed control arm 61 is fixedto the projection of trunnion 53b. The speed control arm 61 isinterlocked with a forward running pedal 8F and a rearward running pedal8R (refer to FIGS. 28 and 29) disposed on the step 4 through aconnecting rod 125 to be discussed below.

In FIG. 3, a motor mounting surface 54 is formed on the lower outersurface of the center section 40. On the motor mounting surface 54 isrotatably disposed a cylinder block 55. An output shaft 45 engages withthe central portion of the cylinder block 55. A plurality of axialpistons 56 are housed in a plurality of cylinder bores in the cylinderblock 55. Through biasing springs the axial pistons 56 are movable inreciprocation, and abut at the heads against a fixed swash plate 57. Thefixed swash plate 57 and cylinder block 55 are housed in a motor casing58 mounted onto the outer surface of the center section 40.Consequently, the lower inner surface of the center section 40 at theside opposite to the motor mounting surface 54 is vacant, therebycreating a wide space in the first chamber R1 for housing a restraintmechanism for high speed rearward movement and a front wheel drivingclutch mechanism C.

The hydraulic pump P1 and hydraulic motor M are fluidly connected witheach other through a pair of oil passages 40a and 40b (refer to FIG. 9)so as to form a closed fluid circuit into which pressurized oil issupplied from charging pump P2 provided on the input shaft 12. As shownin FIGS. 8, 10 and 11, a filter mounting portion 10g for detachablymounting an oil filter 46 to the right side wall of the front casing 10is formed at the lower outer surface of the casing 10. An oil bore 46aformed in the filter mounting portion 10g and communicating with aninlet port of the oil filter 46, as shown in FIG. 8, communicates withthe second chamber R2 through an oil bore 120 open at the rear wall 10b.Lubricating oil stored in the second chamber R2 is guided into the oilfilter 46 from the oil bore 120 through the oil bore 46a. Afterfiltration by the oil filter 46, oil is guided from an outlet port 46b(refer to FIG. 10) of the oil filter 46 through an oil passage 121a,provided at a thick portion of the right side wall of the front casing10, to a suction port of the charging pump P2. An oil passage 121b isformed between the inner surface of the right side plate 60 and theouter surface of the right side wall of the front casing 10. At the topof oil passage 121b, oil passage 121c opens on the front of casing 10,allowing oil to pass to an oil passage in the center section 40.

Thus, the hydraulic pump P1 is housed in the first chamber R1, wherebythe oil stored therein is apt to be at a relatively high temperature. Onthe other hand, the gear-type speed changing unit and a PTO transmissionunit, to be discussed below, are housed only in the second chamber R2,wherein the temperature of oil therein is not so high. Hence, the oil ata relatively low temperature is taken in the charging pump P2 throughthe oil filter 46 and supplied to the closed circuit of the HST,improving the durability of the hydraulic pump and hydraulic motor.Also, elements related to the oil filter 46 are collectively disposed inthe front casing 10, so that short and simple oil passage construction,without piping, can take in the oil within the rear casing 11, therebyreducing the manufacturing cost.

Pressurized oil discharged from a discharge port of the charging pumpP2, as shown in FIG. 12, is set by a main relief valve 47. Part of thepressurized oil is sent to a directional control valve 200 for powersteering through the piping and to a resistance valve 48 housed in thecasing 105 for the charging pump P2. When the directional control valve200 is switched by the rotation of the handle 3, oil is sent to advanceand retract a chamber of a double action hydraulic cylinder 22, therebysteering the front wheels. The return oil from the steering cylinder 22is returned to an oil cooler 123 and a motor casing 58 through thepiping, and further sequentially flows into the first chamber R1 throughan oil bore at the center section 40 and into the second chamber R2through an oil bore at the rear wall 10b. The pressure of the oildischarged from the charging pump P2 is adjusted by a pressure reducingvalve 49 housed in the casing 105 for the charging pump P2. The oilpressure opens one of the check valve 124 at the low pressure side tosupply oil to either oil passage 40a or 40b in the closed circuit. Whenthe oil pressure at the high pressure side oil passage 40a becomeshigher than a set pressure, a high pressure relief valve 104 isactuated. Drain oil produced when the pressure reducing valve 49 isactuated for pressure adjustment, is sent to a PTO clutch brake controlvalve 101.

The oil passages 40a and 40b constitute a closed circuit. As shown inFIG. 9, oil passages 40a and 40b connect with check valves 102--havingoil filters 103 contained in the front wall 10a of the front casing 10.If a vehicle stops its engine E on a slope and the oil in the closedcircuit leaks from hydraulic motor M or hydraulic pump P1 to cause adecrease in operating oil, the lubricating oil in the first chamber R1is supplied by self suction, due to negative pressure, into the closedcircuit through the oil filters 103 and check valves 102. As shown inFIG.3, the main relief valve 47 is situated at the upper portion ofcenter section 40 for setting the discharge oil pressure of the chargingpump P2 at the specified value.

Next, as shown in FIGS. 2, 4 and 5, a PTO counter shaft 39, a PTOtransmission shaft 33, a second running transmission shaft 42 and athird running transmission shaft 43 are journalled between the rear wall10b of the front casing 10 and the partition 11a in the rear casing 11longitudinally parallel to each other through bearings. Enlargedportions 10i and 11b downwardly and slantingly project from the jointportion between the front casing 10 and the rear casing 11. A countershaft 99 and a mid-PTO shaft 14 are rotatably and longitudinallyjournalled in the swollen portions 10i and 11b through bearings,respectively.

Also, as shown in FIG. 5, at the upper portion in the third chamber R3 arear PTO driving shaft 35 is longitudinally journalled. Rear PTO drivingshaft 35 is coaxial with the PTO transmission shaft 33. At the bottom inthe third chamber R3 are juxtaposed a differential gear unit D and anoil filter 38 for the hydraulic lift. The rear PTO driving shaft 35enters at the rear end thereof into the fourth chamber R4 and a gear 35ais engraved onto the extension entering therein. In the fourth chamberR4, between the rear wall 11c and the lid 79, a rear PTO shaft 36 isjournalled. Within the fourth chamber R4, a gear 37 is mounted on shaft36 so that the gear 35a engages with a gear 37. The rear PTO shaft 36projects outwardly from the rear of the housing through the lid 79.

Next, explanation concerns will be given on a power transmissionmechanism of the transmission in accordance with FIGS. 2 through 7. Asmentioned above, power is transmitted to the input shaft 12 from theengine E through a damper and coupling shaft 16. The hydraulic pump P1and charging pump P2 are driven by the input shaft 12 and oil pressurefrom the hydraulic pump P1 is sent to the hydraulic motor M so as tosteplessly drive the output shaft 45. The first running transmissionshaft 41 is integrally spline-coupled with the rear end of the outputshaft 45. A gear 62 is fixed to the rear end of the shaft 41 perforatingthe rear wall 10b.

Next, explanation will be given on the gear-type speed changing unit. Asshown in FIGS. 2 and 4, the gear 62 on the first running transmissionshaft 41 engages with a larger diameter gear 63 fixed onto the secondrunning transmission shaft 42. The larger diameter gear 63 permanentlyengages with a smaller diameter gear 65a, which is freely rotatablyfitted onto the third running transmission shaft 43. A smaller diametergear 64 is fixed onto the second running transmission shaft 42 andpermanently engages with a larger diameter gear 66a. Gear 66a is freelyrotatably fitted onto the third running transmission shaft 43. A hub 67is fixed on the third running transmission shaft 43 between the smallerdiameter gear 65a and the larger diameter gear 66a. A clutch slider 68is non-relatively rotatably and axially slidably spline-fitted onto thehub 67.

A shift fork 106, as shown in FIG. 9, is retained by an annular recess68a formed on the outer periphery of the clutch slider 68. The shiftfork 106 is fixed to a shifter shaft 107, and the front portion thereofis interlocked with a rearward speed change restraint mechanism of aspeed control arm 61 to be discussed below. The rear portion of theshifter shaft 107 engages with a pin projecting from an arm 108. The arm108 is fixed to the inner end of a switching shaft 109 pivotallysupported to the left side wall of the rear casing 11. At one outer endof the switching shaft 109 is fixed a high-low ("H-L") switching arm110, which is connected with the aforesaid sub-speed change lever 7through a link or the like.

Engaging members 65b and 66b provided on the gears 65a and 66a arerespectively formed to be engageable with the internal teeth of theclutch slider 68. Therefore, when the sub-speed-change lever 7 isoperated, the clutch slider 68 is axially slidably moved through the H-Lswitching arm 110, switching shaft 109, arm 108, shifter shaft 107 andshifter fork 106, so that either one of the gears 65b or 66b engageswith the third running transmission shaft 43, thereby providing a twohigh-low step speed change.

More particularly, when the sub-speed change lever 7 is shifted to thehigh speed position, the clutch slider 68 engages at the internal teeththereof with the engaging member 65b of the smaller diameter gear 65a,so that the power from the output shaft 45 is transmitted through thefirst running transmission shaft 41 to gear 62, to larger diameter gear63, to smaller diameter gear 65a, to engaging member 65b, to slider 68,to hub 67, to third running shaft 43 and power of the high speedrotation is transmitted from a bevel pinion 69 provided on the rear endof the third running transmission shaft 43 to the axle 28 through thedifferential gear unit D.

When the sub-speed-change lever 7 is operated in the low speed position,the clutch slider 68 engages at the internal teeth thereof with theengaging member 66b of the larger diameter gear 66a, so that power fromthe output shaft 45 is transmitted through the first runningtransmission shaft 41, to gear 62, to larger diameter gear 63, to secondrunning transmission shaft 42, to smaller diameter gear 64, to largerdiameter gear 66a, to engaging member 66b, to clutch slider 68, to hub67, to third running transmission shaft 43, thereby transmitting thepower of low speed rotation to the axle 28, the same as mentioned above.

Next, explanation will be given on a restraint mechanism which restrainsthe speed control arm 61 when the rearward speed is changed. As shown inFIGS. 3, 8 and 9, the shifter shaft 107 extends at the front end intothe first chamber R1. At the front end of the shifter shaft 107 isformed a cam surface cut semicircularly when viewed in section andhaving the deepest portion 107a. A restraint arm 111 is disposed belowthe hydraulic pump P1. The restraint arm 111 has a first arm 111a and asecond arm 111b which extend in different directions from each other soas to be nearly V-like-shaped, and are fixed at the middle portion tothe support shaft 113. The support shaft 113 is journalled between thecenter section 40 and the rear wall 10b of the front casing 10 andlongitudinally axially of the vehicle body, thereby laterally swingablysupporting the restraint arm 111.

A cam 112, provided at the utmost end of the first arm portion 111a ofthe restraint arm 111, abuts against the cam surface of the shiftershaft 107. The second arm portion 111b abuts at the utmost end thereofagainst one end surface of a restraint pin 114. The restraint pin 114 ispositioned on the outer surface of the right side plate 60 andsubstantially at an intermediate portion of a rearward movement sidespeed change rotation range (RB) of the speed control arm 61, as shownin FIG. 20. The restraint pin 114 is pivotally supported by the sideplate 60 so that it may inwardly and outwardly advance and retractparallel to the axis of slanting rotation. A spring 116 is fitted on theouter periphery of the restraint pin 114 positioned in the first chamberR1 so as to bias the restraint shaft 114 to retract into the firstchamber R1.

Thus, the restraint mechanism is contained in the first chamber R1 forthe HST, whereby a space in the first chamber R1 can effectively beutilized to reasonably dispose the restraint mechanism and impracticalhigh speed rearward movement can automatically be restrained. Also,since the restraint mechanism is contained in the first chamber R1,there is little interference from foreign objects, such as dust, mud orrust, and the restraint mechanism stably accurately operates for a longtime, thereby improving the durability and reliability. Thisconstruction requires fewer parts, allowing for more efficient andcheaper manufacture.

As shown in FIGS. 17 through 20, a speed control arm 61 is fixed ontothe utmost end of a trunnion 53b and projects from the right side plate60. On the rotating base side of speed control arm 61 is a retainingportion 61a, which is disposed within a U-Like-shaped stopper 60a andmounted on the outer surface of the right side plate 60. When theretaining portion 61a rotates to abut against the inside surface of therecess of the stopper 60a, the maximum forward accelerating position Fand the maximum rearward accelerating position R are set. At the utmostend of the speed control arm 61 are mounted a connecting rod 125 and atelescopic member 126a for a shock absorber 126. The connecting rod 125is interlocked with the forward running pedal 8F and rearward runningpedal 8R which arc disposed on the step 4. The shock absorber 126 isswingably pivoted at the base end 126b thereof onto the outer surface ofthe right side wall, and the telescopic member 126a prevents the speedcontrol arm 61 from abrupt rotation.

When the sub-speed-change lever 7 is turned to the neutral position andthe low speed position, as shown in FIG. 8, the pin 112 is positioned onthe deepest position 107a of cam surface, the restraint arm 111 is keptin the state as shown, and the speed control arm 61, as shown in FIGS.18 and 20, can freely rotate within the entire ranges of the forwardmovement side speed change rotation (FB) and of rearward movement sidespeed change rotation (RB). When the sub-speed-change lever 7 is turnedto the high speed position, the shifter shaft 107 slides forward andpushes the cam 112 at the utmost end of the first arm portion 111a ofthe restraint arm 111 whereby the restraint arm 111 swingscounterclockwise in FIG. 8 around the support shaft 113 and the secondarm portion 111b pushes the inner end surface of the restraint pin 114to allow the restraint pin 114 to project outward from the outer surfaceof the right side plate 60.

Accordingly, when the speed control arm 61 is intended to rotate towardthe rearward movement side from the neutral position (refer to FIG. 17)and reaches a substantially intermediate portion of the rearwardmovement side rotation range as shown in FIG. 19, one side surface ofspeed control arm 61 contacts with the outer periphery of the restraintpin 114. Thus, rearward side speed change rotation (RB') is restrictedand acceleration limited. In brief, when the sub-speed-change lever 7 isswitched to the high speed side and the rearward running pedal 8R istrod, the vehicle body rearwardly moves at high speed, which isinconvenient for use. Therefore, the restraint pin 114 automaticallycuts the useless high speed rearward movement. The rearward accelerationis performable over the entire rotation range (RB) of the speed controlarm 61 only if the sub-speed-change lever 7 is turned toward the lowspeed side.

Next, explanation will be given on the power transmission mechanism fordriving the front wheels. As shown in FIGS. 2, 3, 4, 5, and 7, a gear71, fixed to the front end of the third running transmission shaft 43,permanently engages a gear 70 which is rotatably fitted to the front endof the second running transmission shaft 42. The gear 70 permanentlyalso engaging with a gear 72 fixed onto a running power take-out shaft44 (refer to FIGS. 3, 5 and 7). A front wheel driving shaft 13, which isrotatably supported onto the front wall 10a through a bearing, and therunning power takeout shaft 44, which is rotatably supported to the rearwall 10b through a bearing, are coaxially fitted at the front end ofpower takeout shaft 44 and the rear end of the front wheel driving shaft13 in the first chamber R1.

A clutch mechanism C, for engaging and disengaging the front wheel driveis provided between the front end of the running power take-out shaft 44and the rear end of the front wheel driving shaft 13. In detail, splinesare formed on the outer periphery of the front end of the running powertake-out shaft 44 and on the outer periphery of the rear end of thedriving shaft 13. On the splines at the rear end of the front wheeldriving shaft 13 is fitted a clutch slider 73 not relatively rotatablyand axially slidable. As shown in FIG. 8, an annular grove 73a is formedat the outer periphery of clutch slider 73. The shift arm 74, at thelower end is retained by the annular groove 73a, and, at the upper end,is fixed to the inner end of a shifter shaft 75. The shifter shaft 75 ispivoted onto the left side plate 77, which closes an opening formed onthe outer surface of the left side wall of the front casing. Fixed atthe outer end of the shifter shaft 75 is an on-off operating arm 76.Operating arm 76, thus, is connected to the side of the transmissionopposite to the speed control arm 61. The on-off arm 76 is, in turn,connected to a front wheel driving on-off lever (not shown) provided ona driver's cab through a link or the like. Therefore, the speed controlarm 61 and operating arm 76, which are disposed opposite to each otherwith respect to the housing, can easily interlock with the speed changeoperating member and the front wheel on-off operating member in the cabwithout any interference. In addition, a ball detent mechanism isprovided between the clutch slider 73 and the front wheel driving shaft13 for enabling the clutch slider 73 to be held in the positions of"4WD" and "2WD" respectively.

In such construction, when the on-off operating arm 76 is turned to the"4WD" position, the clutch slider 73 couples the front wheel drivingshaft 13 with the running power takeout shaft 44. Consequently, thepower from the third running transmission shaft 43 is transmittedthrough the gear 71, to freely fitted gear 70, to gear 72, to runningpower takeout shaft 44, to slider 73, to front wheel driving shaft 13,to universal joint 17, to input shaft 19, to front axle casing 18 todrive the front wheels 20. Simultaneously, the third runningtransmission shaft 43 drives the rear wheels 21 through the differentialgear unit D, so that the vehicle is driven by four wheel drive (4WD).When the on-off operating arm 76 is turned to the "2WD" position, theclutch slider 73 disconnects the running power takeout shaft 44 from thefirst wheel driving shaft 13, thereby performing two wheel drive (2WD)only.

As mentioned above, since the front wheel driving shaft 13 is rotatablysupported in the first chamber R1 of the HST housing the front wheeldriving shaft 13 can be supported longitudinally of the vehicle bodyregardless of the gear-type speed change unit and disposed substantiallyin the central position laterally of the vehicle body. Such constructionfacilitates transmission of power from the gear-type speed change unitto the front wheel driving shaft. In addition, no mater whether the gearspeed unit--connected to the output shaft 45 of hydraulic motor M (asdescribed above)--is set in a high or low position, the front and rearwheels will rotate synchronously.

The clutch mechanism C for engaging or disengaging the front wheeldriving shaft 13 and/or running power takeout shaft 44--serving as theoutput portion of the gear-type speed change unit--can be housed in thelower space in the first chamber R1. The running power takeout shaft 44is disposed coaxially and relatively rotatably with respect to the frontwheel driving shaft 13. The respective shafts are journalled by thefront wall 10a and rear wall 10b, constituting the first chamber R1, andthe clutch mechanism C is interlocked between the abutting portion ofboth the shafts 13 and 44. Thus, the clutch mechanism can be constructedsimply and compactly while effectively utilizing, the vacancy in thefirst camber R1.

Next, explanation will be given on the PTO transmission unit. As shownin FIGS. 2 and 5, the rear end of the input shaft 12 perforates throughthe rear wall 10b of the front casing 10 and fixes a gear 80. As shownin FIGS. 2, 5 and 6, the gear 80 engages with a gear 81, which is fixedonto the PTO counter shaft 39. The gear 81 engages with a freely fittedgear 82 rotatable through a bearing 83 on the PTO transmission shaft 33.The gear 82 is provided at the side surface with a boss. As enlarged inFIG.6, between the boss and a clutch casing 84, which is fixed onto thePTO transmission shaft 33, are interposed a plurality of friction platesso as to construct a multi-friction plate type hydraulic PTO clutchmeans 85. The pistons 86 in the clutch casing 84, when slid by supply ofoil pressure to be discussed below, brings the friction plates in presscontact with each other, and allows the cam 82 to engage with the PTOtransmission shaft 33 through the clutch casing 84, so that the power isadapted to be transmitted from the input shaft 12 to the PTOtransmission shaft 33. In addition, reference numeral 89 designates aspring for biasing the piston 86 in the direction of releasing thebiased friction plates.

The PTO transmission shaft 33 projects at the front end thereof throughthe rear wall 10b of the front casing 10 and is positioned in the firstchamber R1. A PTO brake G is constructed therein so as to effectivelyuse the vacancy in the first chamber R1. The PTO brake G, as shown inFIGS. 6 and 21, forms splines 33a on the outer periphery of the frontend of PTO transmission shaft 33 and retains the rotary plates into thesplines 33a. A recessed brake chamber 130 is formed on the front surfaceof the rear wall 10b of front casing 10. On the inner periphery of therecessed brake chamber 130, fixed plates are non-rotatably retained. Thefixed plates and rotary plates are alternately put in layers so as toform a braking friction member 131 which is disposed in the brakechamber 130. A pressure plate is disposed to one side of the brakingfriction member 131.

The brake chamber 130, which is formed in a recessed portion at one sidesurface of the rear wall 10b of the front casing 10 for partitioning thefirst chamber R1 from the second chamber R2, does not need to beseparate from the housing. Moreover, because, the braking frictionmember 131 is contained in the brake chamber 130 and biased by thepressure plate 90, the PTO brake G can be small in size. Therefore, thespace it occupies in the interior of the first chamber R1 can bediminished.

The pressure plate 90 is disc-like in shape. At the center of the rearsurface of pressure plate 90 is a biasing portion 90a projecting towardthe braking friction member 131. Coaxially projecting from the annularportion of pressure plate 90 is and an annular piston portion 90b whichextends in the same direction as the biasing portion 90a. The pistonportion 90b is larger in diameter than the braking friction member 131and slidably fitted into a cylinder chamber 132 of an annular recess atthe outer periphery of the brake chamber 130 on the front surface of therear wall 10b and positioned on the outer periphery of the brakingfriction member 131. Thus, the pressure plate 90, which consists of theannular piston portion 90b at the outer periphery, and the biasingportion 90a at the central portion, is made smaller in axial length.Thus, the entire length of the PTO brake G is reduced so that thepressure plate 90 can reasonably be housed in a restricted space in thefirst chamber R1. A bell-shaped cover 87 is mounted on the front surfaceof the rear wall 10b of the front casing 10 to cover the pressure plate90. A coil spring 88 housed in a narrow, elongated portion of the cover87 abuts at the base end against the inner bottom surface of the cover87 and at the action end against the surface of the pressure plate 90.Therefore, the pressure plate 90 is biased in the direction of biasingthe braking friction member 131 and within the cover 87.

Oil passages 33b and 33c are bored in the PTO transmission shaft 33. Theoil passage 33b opens at one end into the cylinder chamber of the clutchchamber 84 of the PTO clutch means 85 which houses the pistons 86. Atthe other end of oil passage 33b, it communicates with a rotary joint.The rotary joint connects an annular groove 33d, which is formed on theouter periphery of the axial end of the PTO transmission shaft 33, tothe inner peripheral surface of a through bore, which is in slidablecontact with the groove 33d at the rear wall 10b. An oil passage 10f isbored at the rear wall 10b of the front casing 10 along the thickportion thereof and communicates at one end with the rotary joint and isopen at the other end on the left side surface of the first casing 10.An oil passage 10c, communicating at one end with the cylinder chamber132 of the PTO brake G, is bored in the thick portion of the rear wall10b and perpendicularly communicates at the other end with the oilpassage 10f.

The oil passage 33c in the PTO transmission shaft 33 is open at one endon the front end surface of the PTO transmission 33 shaft andcommunicates with interior of the brake chamber 130 of the PTO brake Gand closed by the cover 87. An oil passage 10e is bored in the thickportion of the rear wall 10b and communicates at one end with the brakechamber 130 and is open at the other end on the left side surface of thefront casing 10.

Thus, the operating oil for the PTO clutch means 85 and the PTO brake Gcan be discharged from the oil passages 10f and 10e formed at the rearwall 10b through the PTO transmission shaft 33 positioned therein.Consequently, the total length of the oil passages is reduced and theconduit resistance is diminished. As a result, the PTO brake G canreliably and quickly be exerted.

FIGS. 13, 14 and 15 show the construction of a left side plate 77, whichis mounted on the outer surface of the left side wall of the frontcasing 10. The shifter shaft 75 and switching shaft 134 are pivotallysupported onto the left side plate 77. Also on left side plate 77 isdisposed a cylindrical PTO clutch/brake control valve 101, foralternately switching the PTO clutch 85 in "connection ordisconnection." As shown in FIGS. 13 and 15, four oil grooves 77a, 77b,77c and 77d, are channeled through the surface of the left side plate 77which abuts the front casing 10. When the left side plate 77 is mountedonto the left side surface of the front casing 10, these oil grooves77a, 77b, 77c and 77d form oil passages, wherein: the oil groove 77ccommunicates with a pump port of the PTO clutch/brake control valve 101;the oil groove 77a communicates with a clutch port thereof; and the oilgrooves 77a and 77b communicate with each other through a relief valve119 mounted into the left side plate 77. As a result, the oil flowing inthe oil groove 77a is adjusted in pressure to a specified value by therelief valve 119, and relief oil discharged from the relief valve 119 isadapted to flow in the oil groove 77b.

The left side plate 77 covers the oil passage 10e and 10f, which areopen on the outer surface of the left side plate of the front casing 10.The left side plate 77 lines up to allow the oil groove 77a tocommunicate at one end with the oil passage 10f, and the oil groove 77bwith the oil passage 10e. Also, the oil groove 77d communicates with adrain port of the PTO clutch/brake control valve 101 and also with anoil sump within the front casing 10 through a drain bore 10m (refer toFIG. 22) open at the outer surface of the left side wall of the frontcasing 10. Furthermore, the oil groove 77a, as shown in the right halfof FIG. 10 and in FIG. 22, connects with an accumulator 117, which cancontrol pressure oil flowing from the oil groove 77a to the rotary jointso as to slowly connect the PTO clutch means 85, thereby enabling aworking machine of a large amount of inertia to be properly drivenwithout shock.

The accumulator is constructed as follows: a cylinder chamber 10d isbore into the thick portion of the rear wall 10b of the front casing 10,perpendicularly to the input shaft 12; a spring 117b and a piston 117aare inserted into the cylinder chamber 10d ; the left side plate 77covers the head of the piston 117a, and a pressured chamber is formedbetween the head and the left side plate 77 to communicate with the oilgroove 77a. Hence, a casing for the accumulator utilizes part of thehousing which, in turn, simplifies its construction and allows for a lowmanufacturing cost. Also, the PTO clutch/brake valve 101 and accumulator117 are connected simultaneously with the mounting of left side plate77, thereby enabling the assembly thereof to be simplified. In addition,the cylinder chamber 10d communicates at the spring housing side with abearing chamber 10j for the input shaft 12 so that oil having leaked tothe spring containing side lubricates the bearing in the bearing chamber10j of the input shaft 12.

The PTO clutch brake control valve 101 comprises a 3-port 2-positiondirectional control type solenoid valve. A pump port thereofcommunicates through the oil groove 77c with one end of the oil passage10n (refer to FIGS. 8 and 11) which extends forward along the thickportion of the left side wall of front casing 10. An oil passage 10ncommunicates at the other end with the drain port of the pressurereducing valve 49 housed in the pump casing 105. This communication isthrough a perforating bore (not shown), which is formed at the centersection mounted to the front casing 10. As a result oil discharged fromthe valve 49 is guided into the oil passage 10n. The PTO clutch/brakecontrol valve 101 can alternatively switch a PTO changeover switch (notshown) provided on a dash board 2 to an "engaging position" or"disengaging position" by the ON-OFF operation of the operator. When thePTO clutch/brake control valve 101 is put in "the disengaging position",the state is as shown in FIG.12. In this position, the oil passages 77aand 77c communicate with the oil groove 77d, so that the oil passages10n, 10f and 10e--connecting, respectively, with the drain port of thepressure reducing valve 49, the cylinder chamber of the PTO clutch means85 and the cylinder chamber 132 of the PTO brake G, are open so thatpressurized oil drains through bore 10m and into an oil sump in thefront casing 10 from the oil groove 77d. Hence, since the piston 86 ofthe PTO clutch means 85 is biased by the spring 89 to move away from thefriction plate, the clutch means 85 disconnects to cut off the powertransmission to the PTO transmission shaft 33. Meanwhile, the coilspring 88 of the PTO brake G biases the biasing portion 90a of thepressure plate 90 so as to bias the braking friction member 131 to brakethe PTO transmission shaft 33, thereby quickly stopping inertia rotationand together rotation of the mid-PTO shaft 14 and rear PTO shaft 36, tobe discussed below.

When the PTO clutch brake control valve 101 is put in the "engagingposition", the oil groove 77c communicates with the oil groove 77a andthe oil groove 77d is blocked. Hence, pressurized oil flowing in thedrain port of the pressure reducing valve 49, flows into the cylinderchamber of the PTO clutch means 85 through the oil passage 10f at therear wall 10b of the front casing 10, the rotary joint and the oilpassage in the PTO transmission shaft 33. As a result, the piston 86biases the friction plate to connect the PTO clutch means 85, whichtransmits the rotation of input shaft 12 to the PTO transmission shaft33. On the other hand, the oil pressure, flowing in the oil passage 10f,branches to flow into the oil passage 10c and into the cylinder chamber132 of the PTO brake G so as to move the pressure plate 90 away from thebraking friction members 131 against the coil spring 88, therebyreleasing the braking action to the PTO transmission shaft 33. Inaddition, the relief oil, produced when the oil pressure flowing in theoil groove 77a is adjusted by the relief valve 119, flows into the oilgroove 77a to lubricate the braking friction member 131. It then flowsinto the oil passage 33c from the front end surface of the PTOtransmission shaft 33 to lubricate the friction plate of PTO clutchmeans 85 and the respective lubricated portions, and thereafter isreturned to the oil sump in the rear casing 11.

The relation between the shaft torque and the time is shown in the graphin FIG. 16. Curve (a) measures the brake torque for braking the PTOtransmission shaft 33 by the brake G against time, and curve (b)measures the transmission torque at the PTO transmission shaft 33against time. Before the PTO change-over switch is put in the "notengaged position," at a time (t0), the brake torque, corresponding to atorque value (T1), acts on the PTO transmission shaft 33. When the PTOchange-over switch is switched to the "engaged position," the pressureplate 90 of the PTO brake G is given pressure to move in the directionof weakening the biasing force with respect to the rotary plate. Asdemonstrated by curve (a), the brake torque reduces uniformly with thelapse of time. Meanwhile, until a time (t1), the operating oil has notfilled in the cylinder chamber of the piston 86 of the PTO clutch means85 so as not to generate the transmission torque of the PTO transmissionshaft 33. Thus, until the time (t1), the value of curve (b) along thevertical axis is zero. At the time (t1) the operating oil is filled inthe cylinder chamber and a transmission torque generates in the PTOshaft 33. Thus, at time (t1) curve (b) increases to a positive value.During the time (t1) to (t2), the piston 117a of the accumulator 117 isdisplaced against the spring 117b to fill the operating oil into thecylinder chamber 10d; whereby the oil pressure, acting on the piston 86of the PTO clutch means 85 reaches a level that causes the PTO clutchmeans 85 to be slowly connected while gradually rising the biasing forcewith respect to the friction plate, and the transmission torque gentlyrises. When the transmission torque reaches the time (t2), the piston117a stops displacement. During the time (t2) to (t3), the piston 86strongly pushes the friction plate against the biasing force of spring89 of the PTO clutch means 85. Then, when it reaches the time (t3), theclutch means 85 perfectly connects to apply the set maximum transmissiontorque T2 to the PTO transmission shaft 33.

Next, explanation will be given on the driving system of the mid-PTOshaft 14 and rear PTO shaft 36. As best seen in FIGS. 5 and 6, a mid-PTOdriving gear 91 is freely fitted onto an intermediate portion of the PTOtransmission shaft 33. A hub 92 is fixed at the rear portion of the PTOtransmission shaft 33. Behind hub 92 and coaxially freely fitted to therear end of PTO transmission shaft 33 is a transmission shaft 34, whichconnects with the rear PTO transmission shaft 35. An engaging member 93is formed on the side surface of the mid-PTO driving gear 91 and anengaging member 94 is formed on the front end of the transmission shaft34. A clutch slider 95 is spline-coupled onto the hub 92 so that it isnon-relatively rotatable and axially slidable. A shift fork (not shown)is retained by an annular recess formed on the outer periphery of theclutch slider 95 and fixed onto a shifter shaft 122 shown in FIG. 10.The shifter shaft 122 is longitudinally slidably mounted between therear wall 10a of the front casing 10 and the partition 11a of the rearcasing 11 and positioned at the front end in the first chamber R1.Meanwhile, a switching shaft, 134 is rotatably supported by the leftside plate 77. An arm 133 is fixed to the inner end of the arm 134, andis retained at the front end to a groove of the shifter shaft 122. Atthe outer end of the switching shaft 134 is fixed a PTO switching arm135, which is connected with a PTO switching lever 9 disposed at theside of the operator's cab through a link or the like. The mid-PTOdriving gear 91, as shown in FIGS. 2 and 4, engages with an idle gear 97freely rotatably fitted on the axially intermediate portion of the thirdrunning transmission shaft 43 through bearings 96. The idle gear 97engages with a gear 98 fixed onto the counter shaft 99, and the gear 98engages with a gear 100 fixed to the mid-PTO shaft 14, so that power istransmitted from the mid-PTO driving gear 91 to the mid-PTO shaft 14.Thus, a PTO transmission gear train comprising the mid-PTO driving gear91, idle gear 97, gear 98 and gear 100, for transmitting the power tothe mid-PTO shaft 14, is housed in the second chamber R2 and is madecompact in construction because the idle gear 97 is disposed on one ofshafts constituting the gear-type speed change unit.

Accordingly, the PTO switching lever 9 disposed at the side of theoperator's cab is operated to allow the slider 95 to slide rearwardly inthe drawing plane of FIG. 6. When slid rearward, the slider 95 connectsthe hub 92 with the engaging member 94 on the transmission shaft 34. Asa result, power passing through the PTO clutch means 85 is transmittedin the following order: from the PTO transmission shaft 33, to hub 92,to clutch slider 95, to engaging member 94, to transmission shaft 34,and to rear PTO driving shaft 35, thereby driving the rear PTO shaft 36.When the clutch slider 95 is slidably moved forwardly by one step, theclutch slider 95 connects both the engaging member 93 of the mid-PTOdriving gear 91 and the engaging member 94 of the transmission shaft 34with the hub 92. Therefore, power is transmitted to the rear PTO shaft36, as well as to the following in order: the PTO transmission shaft 33,to hub 92, to clutch slider 95, to engaging member 93, and to mid-PTOdriving gear 91, so as to drive the mid-PTO shaft 14 through the PTOtransmission gear train. Consequently, both the mid-PTO shaft 14 andrear PTO shaft 36 can be driven. Furthermore, the clutch slider 95, whenslid to the forward-most position, connects only the engaging member 93of the mid-PTO driving gear 91 to the hub 92. Thus, the power istransmitted, in order, to the PTO transmission shaft 33 to hub 92, toclutch slider 95, to engaging member 93, and to mid-PTO driving gear 91,so as to drive the mid-PTO shaft 14 only.

A differential gear unit D, housed in the third chamber R3 in the rearcasing 11, as shown in FIG. 23, engages through a bevel gear with theoutput shaft 43 of the gear-type speed changing unit at the protectingportion thereof from the partition 11a. The differential gear unit D hasdifferential yoke shafts 28L and 28R leftwardly and rightwardlyextending therefrom. The differential yoke shafts 28L and 28R arepivotally supported in the rear axle casing 127L and 127R, and connectedat the ends with the rear axles 27L and 27R through the reduction geartrain, respectively.

Within the rear axle casings 127L and 127R are housed running brakeunits 137L and 137R for braking the rear axles 27L and 27R. In detail,the differential yoke shafts 28L and 28R are spline engaged with theinner peripheral ends of a plurality of rotation side friction plates137a. The fixed side friction plates 137b, which contact in layers withthe rotation side friction plates 137a, are non-rotatably retained atthe outer peripheral ends to the inner periphery of openings at the rearaxle casings 127L and 127R. At one end of the side surface of eachoutermost fixed side friction plate 137b is disposed eachring-like-shaped pressure plate 137c, and between the side surface ofthe pressure plates 137c and the side surface of the rear axle casings127L and 127R are disposed cam means 137d for generating axial thrust atthe pressure plate 137c.

Brake operating shafts 137e are pivotally supported to each rear axlecasings 127L and 127R. At the inner end of each brake operating shaft137e is formed a cam surface to abut against a projection of eachpressure plate 137. At the outer end of the brake operating shafts 137aare fixed brake arms 152L and 152R, which connect with left and rightbrake pedals 7L and 7R, respectively. When brake operating shafts 137earc rotated by the brake arms 152L and 152R, the cam surfaces rotate thepressure plate 137c around the differential yoke shafts 28L and 28Rthrough the aforesaid projections, and the thrust generating cam means137d act to allow the pressure plate 137c to bias the friction plates137a and 137b. Thus, the running brake units 137L, and 137R are actuatedto exert the braking action onto the differential yoke shafts 28L and28R, which in turn apply the braking action to the rear axles 27L and27R.

Next, explanation will be given on a brake operating mechanism, in whichon the left side step 4 of the vehicle body frame 15 are juxtaposed aleft turn brake pedal 7L and a right turn pedal 7R. On the right sidestep 4 are juxtaposed a forward running pedal 8F and a rearward runningpedal 8R. A master brake pedal 136 having a wide pedal face is disposedin front of and slightly above both the pedals 8F and 8R.

As shown in FIG. 24, on the vehicle body frame 15, two support pipes 15aand 15b laterally extending are fixed longitudinally of the body frame15 and in parallel to each other. Within the support pipe 15a, a firstbrake pedal shaft 139 is rotatably supported. Brake pedal shaft 139 islonger than the support pipe 15a and therefore projects out both ends ofsupport pipe 15a. Within the support pipe 15b is rotatably supported asecond brake pedal shaft 140, which is likewise longer than support pipe15b and therefore projects from both ends of support pipe 15b. At theleft end of the second brake pedal 140 are juxtaposed a cylindricalmembers 7La and 7Ra. Affixed on cylindrical member 7La is a left turnbrake pedal 7L and affixed on cylindrical member 7Ra is a right turnbrake pedal 7R. Both the cylindrical members 7La and 7Ra are locked ontothe second brake pedal shaft 140 by a snap ring 142. The left turn brakepedal 7L and right turn brake pedal 7R project forwardly and upwardly ata slant forming a U-like bend, so that the pedal faces are positioned onthe same plane with each other. The cylinder member 7La is relativelyrotatably fitted and has on the outer periphery, an arm 143L and anabutting segment 144L which radially project from the cylinder member7La. A link 145L is connected at the one end with the arm 143L and atthe other end with a brake arm 152L of the brake unit 137L, as shown inFIG. 23.

The cylindrical member 7Ra positioned inside the cylindrical member 7Lais non-relatively rotatably coupled with second brake pedal shaft 140and only an abutting segment 144R projects from the outer periphery ofthe cylindrical member 7Ra. The abutting segments 144L and 144R projectrearwardly and downwardly at a slant and allow both the abuttingsurfaces to be positioned in the same plane. A cylindrical member 7Rb isnon-relatively rotatably coupled with the right hand end of the secondbrake pedal shaft 140. An arm 143R projects from the cylindrical member7Rb, and a link 145R is connected at one end thereof with the arm 143Rand at the other end with a brake arm 152R of the running brake unit137R. Between the arms 143L and 143R and a retaining portion (not shown)at the lower surface of left and right steps are stretched tensionsprings 141L and 141R. The tension springs 141L and 141R bias the brakearms 152L and 152R of the brake units 137L and 137R in the direction ofbrake releasing. When either the left hand turn brake pedal 7L or theright hand turn brake pedal 7R--in the not-treading state in FIG. 27--istrod the respective cylinder members 7La and 7Ra rotate tocounterclockwise swing the arms 143L and 143R, so that the links 145Land 145R are pulled forward. As a result, the tension springs 141L and141R expand and the left and right running brake units 137L and 137R areengaged independently through the brake arms 152L and 152R. Therefore,the rear wheels 27L and 27R can be braked independently to assist thevehicle in running.

As shown in FIG. 25, at the left end of the first brake pedal shaft 139a simultaneous brake operating mechanism 150 is provided, which operatesthe left hand turn brake pedal 7L and right hand turn brake pedal 7Rsimultaneously so as to simultaneously actuate the left and right handrear axles 27L and 27R. Namely, a cylindrical member 151a is notrelatively rotatably coupled with the left end of the first brake pedalshaft 139 and an abutting arm 151, substantially L-like-shaped whenviewed in plan, is fixed to the cylindrical member 151a. The abuttingarm 151 is horizontally bent at the utmost end thereof to form a leg151b across the abutting segments 144L and 144R. On the leg 151b areadjusting bolts 153, which are optionally adjustable to fill a gapbetween the tip of each adjusting bolt 153 and the lower surface of theabutting segment 144L or 144R.

Meanwhile, as shown in FIG. 25, the right end of the first brake pedalshaft 139 is non-relatively rotatably coupled with a cylindrical member154. In the cylindrical member 154 is mounted a bracket 155. From theoutside surface of the bracket 55 projects a pin 155a, which extendsparallel to the first brake pedal shaft 139. A cylindrical member 136a,which is fixed to the base of master brake pedal 136, is sleeved andconnected onto the pin 155a. Accordingly, when the master brake pedal136 shown in FIG. 28 is trod, the first brake pedal shaft 139 rotatescounterclockwise through the mounting bracket 155. In turn, the abuttingarm 151 rotates counterclockwise, when viewed in FIG. 28, and theadjusting bolts 153 push the lower surface of the abutting segments 144Land 144R respectively to allow the cylindrical members 7La and 7Ra torotate counterclockwise. Consequently, the left and right turn brakepedals 7L and 7R are simultaneously manipulated so as to simultaneouslyactuate the left and right running brake units 137L and 137R, therebystopping the vehicle.

The first brake pedal shaft 139, and the second brake pedal shaft 140extend laterally of the vehicle body 15 and are positionedlongitudinally thereof. At one lateral end of the second brake pedalshaft 140, are rotatably supported the turn brake pedals 7L and 7R in ajuxtaposed fashion for independently desirably operating the left andright running brake units 137L and 137R which independently brake theleft and right axles. At the opposite lateral end of the first brakepedal shaft 139 is disposed the master brake pedal 136 and at the onelateral end of the same is disposed a simultaneous brake operatingmechanism with respect to the turn brake pedals 7L and 7R. Thus, theshaft for supporting the left and right turn brake pedals 7L and 7R andthat for supporting the master brake 136 are separate from each otherand have no interrelationship. Consequently, when the vehicle body turnsor stops, the desired left and right brake units can stably and reliablybe operated.

The simultaneous brake operating mechanism for the left an right turnbrake pedals 7L and 7R, is constructed so that the abutting arm 151,which has the leg 151b enabling it to simultaneously abut against theabutting segments 144L and 144R provided at the rotary bases of the turnbrake pedals 7L and 7R, is fixed to one end of the first brake pedal 139for supporting the master brake pedal 136. Thus, the mechanism forsimultaneously actuating the left and right running brake units issimple in construction, low in manufacturing cost and simple inassembly.

At the portion of the abutting arm 151 opposite to the abutting segments144L and 144R is provided a mechanism for adjusting the timing ofabutting arm 151 against the abutting segments, whereby, even when anerror is made during assembly, the error can easily be adjusted afterassembly so as to enable the timing to be easily adjusted and corrected,thereby reducing the proportion of defective products. Also, even whenone side running brake unit exerts braking action in advance of theother side, causing wearing or deformation, despite the master brakepedal, the brake units can easily be adjusted and corrected to therebysimplify maintenance thereof.

Next, explanation will be given on an operating mechanism forcontrolling the traveling direction and vehicle speed. As shown in FIG.29, a frame 160 which is laterally and longitudinally defined byelongate plates is mounted at the right side surface of the vehicleframe 15 and below the right side step 4. A first running pedal shaft161 and a second running pedal shaft 163 are pivotally supported to theframe 160 longitudinally of the body and in parallel to each other.

The first running pedal shaft 161 is non-relatively rotatably coupled atone end with a cylindrical member 8Fa to which is fixed the base of theforward running pedal 8F. The second running pedal shaft 163 isnon-relatively rotatably coupled at one end thereof with a cylindricalmember 8Ra to which is fixed the base of the rearward running pedal 8R.To the first running pedal shaft 161 is fixed an arm 164 having a firstleg and a second leg bent in a U-Like shape and is Z-Like-shaped as awhole when viewed from the lateral side (refer to FIG. 28). A roller 166is rotatably supported to a pivot pin 164a, which is provided at thefirst leg of the arm 164 (refer to FIG. 30). To the second running pedalshaft 163 is fixed an arm 165 provided with an abutting portion 165awhich abuts the roller 166. A spring 167 is stretched between the arm165 and the pivot pin 164a so as to bias the abutting portion 165a ofthe arm 165 to permanently abut against the roller 166. A connecting rod125 is connected at one end thereof to the second leg of the arm 164 sothat the arm 164 connect with the speed control arm 61 through theconnecting rod 125.

When the forward running pedal 8F shown in FIG. 28 is trod duringoperation, the arm 164 rotates clockwise around the first running pedalshaft 161, and the connecting rod 125 is pulled forwardly of the vehiclebody so that the speed control arm 61 swings forwardly from the neutralposition shown so as to increase the forward speed (as described above).Also, when the rearward running pedal 8R is trod, the arm 165 swingsclockwise around the second running pedal shaft 163. As a result, theabutting portion 165a pushes the arm 164 counterclockwise, so that theconnecting rod 125 is pushed rearwardly of the vehicle body.Consequently, the speed control arm 61 rearwardly swings from theneutral position shown so as to increase the speed of rearward movementof the vehicle (as described above). In addition, when the operator'sfoot is released from the rearward running pedal 8R, the speed controlarm 61, forward running pedal 8F and rearward running pedal 8R aresubjected to the action of the neutral position biasing spring 59 housedin the front casing 10 and hydraulic reaction of the hydraulic pumphoused in the same so as to automatically return to the neutral positionshown in FIG.28. In addition, as shown in FIGS. 28, and 18 through 20, ashock absorber 126 is mounted to the speed control arm 61 to therebyapply resistance thereto when returning to the neutral position.

Next, explanation will be given on a locking mechanism which can fix theforward running pedal 8F in an optional forward movement operatingposition. In FIG. 28, the second leg of the arm 164 is forwardly bent atthe utmost end. A ratchet 170 is mounted to the upper portion of thesecond leg of the arm 164. On the upper surface of the ratchet 170 are anumber of teeth, formed in a row longitudinally of the vehicle. From theoutside surface of the body frame 15, in front of the ratchet 170,projects a spindle 176, to which a locking arm 174 is pivotallysupported at about the central portion. A pawl 174a is formed at thelower side of the utmost end of the locking arm 174 and disposed abovethe ratchet 170 so that the pawl 174a can engage with the ratchet 170.

A link 175, which is connected to one end of the locking arm 174, isconnected through a lost-motion mechanism 181 to a cruise control lever186 disposed at the rear surface of the dash board 2. The lost-motionmechanism 181, as shown in FIG. 31, is constructed so that a cylindricalmember 186b is fixed to the lower end of the link 186a, which connectswith the cruise control lever 186. The link 175 is axially slidablyinserted at its upper end into the cylindrical member 186b, and atension spring 182 is stretched between the link 186a and the link 175so as to generate a biasing force only in the direction of moving thelinks 186a and 175 toward each other.

Again referring to FIG. 28, a recessed abutting portion 174b is formedon the upper surface of the other end of the locking arm 174. On themounting bracket 155, which as described above supports the master brakepedal 136, are erected a lock releasing pin 156a and stopper pin 156b,which project toward the locking arm 174. The lock release pin 156a ispositioned above the abutting portion 174b. A tension spring 174c isstretched between the utmost end of the forward portion of locking arm174 and the retaining portion provided on the outside surface of thebody frame 15 so as to bias the locking arm 174 to lift the pawl 174a atthe one end thereof away from the ratchet 170 and to abut at the otherend against the stopper pin 156b.

As shown in FIG. 31, a lever guide 183 is attached to the rear surfaceof the dash board 2. The lever guide 183 has a hooked guide groovecontinuously connecting a vertical short lock releasing groove 183a anda vertical long locking groove 183b through a horizontal groove. Whenthe cruise control lever 186 is held in the lock releasing groove 183a,the link 175 is pulled upward through the lost-motion mechanism 181 sothat the locking arm 174 rotates clockwise (when viewed in FIG. 28)around the spindle 176 to disengage the pawl 174a from the ratchet 170.Hence, the forward running pedal 8F or the rearward running pedal 8R canfreely be trod, so that the speed of the HST may enable forward movementor change the vehicle speed. In addition, when the cruise control lever186 is put at the lock releasing portion 183a, rotation of the lockingarm 174 is regulated by the stopper pin 156b, whereby the tension spring182 of the lost-motion mechanism 181 is stretched to generate a biasingforce.

When the operator shifts the cruise control lever 186 to the lockinggroove 183b of the lever guide 183, the biasing force following therestoration of tension spring 182 automatically moves the cruise controllever 186 downwardly to substantially the central position of thelocking groove 183b . In addition, the link 186a abuts against the link175. Thus, the operator further downwardly pushes the cruise controllever 186, whereby the force thereof, as shown in FIG. 28, istransmitted from the utmost end of the link 186a to the locking arm 174through the link 175 so as to rotate the locking arm 174counterclockwise around the spindle 176, whereby the pawl 174a engageswith the predetermined tooth of the ratchet 170. Because the arm 164 issubjected to a counterclockwise force around the first running pedalshaft 161 by the aforesaid neutral braking spring 59 and the hydraulicreaction of the hydraulic pump P1, a predetermined frictional holdingforce is generated at the engaging portion of the pawl 174a with theteeth of ratchet 170 according to the form setting of both the members.Therefore, even when the operator releases his hand from the cruisecontrol lever 186, the engaging state of pawl 174a is maintained. Thus,the forward running pedal 8F is fixed in the predetermined forwardmovement operating position so that, even when the operator removes hisfoot from the forward running pedal 8F, the HST is continuously drivento keep the vehicle at a constant speed.

When the master brake pedal 136 is trod to operate the left and rightbrake units 137L and 137R for halting the vehicle, the lock releasingpin 156a, which projects from the mounting bracket 155, shifts clockwisearound the first brake pedal shaft 139 in FIG. 28 and abuts against theabutting portion 174b of the locking arm 174. The locking arm 174 isthen subjected to the forces of lock releasing pin 156a and tensionspring 174c so as to be rotated clockwise around the spindle 176,whereby the pedal 174a is forcibly disengaged from the teeth of theratchet 170. Hence, the forward running pedal 8F is unlocked and thespeed control arm 61 for the HST and the forward running pedal 8F aresubjected to the force of neutral biasing spring 59 and the hydraulicreaction of the hydraulic pump P1 so as to return to the centralpositions, thereby stopping the vehicle without stopping the engine E.When the locking arm 74 is rotated clockwise, the link 175 together withthe link 186a is upwardly pushed and cruise control lever 186 is pushedupwardly to substantially the neutral position of the locking groove183b of the lever guide 183. Hence, when the vehicle again travelsforwardly, the operator treads the forward running pedal 8F until apredetermined vehicle speed is obtained and the cruise control lever 186is once more pushed down to the lower end of the locking groove 183b ofthe lever guide 183, thereby enabling a constant vehicle speed to againbe maintained.

Thus, the shaft for supporting the left and right turn brake pedals 7Land 7B and that for supporting the master brake pedal 136 are separatelydisposed. Moreover, the lock releasing mechanism for enabling therunning pedals 8L and 8R to be fixed at a set forward speed is providedin the vicinity of the master brake pedal 136 so that the master brakepedal can act to release the mechanism. Thus, the master brake pedal136, when laid out, is easy to design without any limitation, therebyenabling the master brake pedal 136 to be laid out in an optimumposition for operability.

While a preferred embodiment of the invention has been described usingspecific terms, such description is for illustrative purpose only, andit is to be understood that changes and variations may be made withoutdeparting from the spirit of the invention, as defined in the followingclaims.

What is claimed is:
 1. A transmission for a vehicle, comprising:ahousing having a first chamber and a second chamber; a hydrostatictransmission including a hydraulic pump and hydraulic motor wherein oneof said hydraulic pump and said hydraulic motor is housed in said firstchamber and the other of said hydraulic pump and said hydraulic motor isdisposed outside of said first chamber; a gear-type speed changing unitfor changing the running speed of said hydrostatic transmission in aplurality of steps connected in a driving manner to said hydrostatictransmission and housed in said second chamber; and a front wheeldriving shaft drivingly connected to said gear-type speed changing unitrotatably supported in said first chamber and perforating through a wallin said housing and projecting therethrough.
 2. A transmission for avehicle as set forth in claim 1, further comprising:a clutch mechanismdisposed in said first chamber for engaging and disengaging said frontwheel driving shaft with an output portion of said gear-type speedchanging unit.
 3. A transmission for a vehicle as set forth in claim 2,further comprising:a speed control arm for said hydrostatic transmissiondisposed at one side of said housing; and an operating arm for saidclutch mechanism disposed at another side of said housing.
 4. Atransmission for a vehicle as set forth in claim 1, wherein a front wallof said housing for mounting said hydraulic pump and said hydraulicmotor is separable from said housing.
 5. A transmission for a vehiclecomprising:a housing having a first chamber and a second chamber; ahydrostatic transmission switchable between forward and rearwardmovements, said hydrostatic transmission including a hydraulic pump anda hydraulic motor wherein one of said hydraulic pump and said hydraulicmotor is housed in said first chamber of said housing and the other ofsaid hydraulic pump and said hydraulic motor is housed outside saidfirst chamber; a gear-type speed changing unit for changing runningspeed of said hydrostatic transmission in a plurality of steps connectedin a driving manner to said hydrostatic transmission and housed in saidsecond chamber; a speed control arm; and a restraint mechanism housed insaid first chamber for restraining rearward movement of the said speedcontrol arm to no more than a predetermined speed.
 6. A transmission fora vehicle as set forth in claim 5, in which said restraint mechanismfurther comprises:a cam means disposed in said first chamber anddisplaced longitudinally of the vehicle body and interlocking withswitching operation of said gear-type speed changing unit; a restraintpin provided at a predetermined position along a rearward movement sideof a speed changing range of said speed control arm wherein saidrestrain pin projects and retracts with respect to said speed controlarm; and a restraint arm supported in said first chamber and pivotableabout a longitudinal axis of said vehicle body including a first armportion abutting against said cam means and a second arm portionabutting against one end of said restraint pin, whereby when saidgear-type speed changing unit is switched to the high speed stage, saidfirst arm portion rides on a thin portion of said cam means so as toswing said restraint arm and whereby said second arm portion is actuatedto push said restraint pin toward said speed control arm.
 7. Atransmission for a vehicle as set forth in claim 5, wherein:a front wallof said housing for mounting said hydraulic pump and said hydraulicmotor is separable from said housing.
 8. A system for transmitting powerfrom an engine to driving wheels of a vehicle, comprising:a housing; ahydrostatic transmission switchable between forward and rearwardmovements disposed in said housing; a gear-type speed changing unitincluding a speed changing member for changing the running speed of saidhydrostatic transmission in a plurality of steps; and a restraintmechanism provided on said housing for limiting rearward acceleration ofthe driving wheels to no more than a predetermined speed by limitingmotion of said speed changing member, when said gear-type speed changingunit is positioned at a high speed setting.